Why does the piston to head clearance decrease with heat?

[Tippy]

As I was pondering, if you have a steel crank and steel connecting rods, why on a 911 engine does the piston to head clearance decrease with heat?

You'd think the cases are growing, the cylinders are growing, and they would expand farther than the rod and crankshaft made of steel radially.

Sorry, too much caffeine and my mind started wandering.....

[JoeMag]

I had a discussion with well known 911 engine builder and he was telling me about engines that he’d built that had zero piston to valve clearance so the could not run them higher than idle until they warmed up and they’d have clearance. He noted a 911 engine grows 1/8” wider from cold to NOT.

So I would assume piston to head clearance would grow too.

[Tippy]

Originally Posted by JoeMag

I had a discussion with well known 911 engine builder and he was telling me about engines that he’d built that had zero piston to valve clearance so the could not run them higher than idle until they warmed up and they’d have clearance. He noted a 911 engine grows 1/8” wider from cold to NOT.

So I would assume piston to head clearance would grow too.

Hmmm, other engines you try to achieve zero clearance but I assumed we had put in clearance because the pistons could touch?

[cgarr]

All depends on what parts are made from that's why some motors you set the valve lash hot and some cold..

[Speedy Squirrel]

When I was doing it, it was for tolerance stackup, plus any necessary adjustments for desired compression ratio and carbon buildup.

Crank centerline maximum unfavorable offset
Main and rod minimum diameters
Rod length at maximum
Piston compression height at maximum

I don’t think it does change too much when warm.

[Ollies930]

The main reason for the piston to head clearance is to compensate for rod stretch, piston pins bending, crankshafts bending and piston rock. The higher the rpm, the more this happens, which is why piston to head clearance has to take rpm into account. As for engines growing due to heat expansion, I have heard many anecdotes of how much engines grow. In reality, if an engine were to grow as much as some people claim, then the bearings would be falling out of those engines.

[targa72e]

1/8" is a little large. Closer to 1/16" . If we take a look at the thermal expansion rate of Aluminum and pick (22 ×10-6/°C)

https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

I would estimate that a engine is about 3 feet wide quick calculation shows about .22mm growth for 100Deg C change.

https://goodcalculators.com/thermal-expansion-calculator/

Not all the parts of the engine will heat up the same with heads having the biggest temp change change. So a 100 DEG c change is probably a little low.

john

[Speedy Squirrel]

I believe that if the aluminum elongated by 1/8” that would make the piston to head clearance increase and a cold engine would be the worst case, since the steel crank and rods are not expanding at that rate.

The engine is constrained in all the right places by steel. That is why I keep harping on using the right head bolts. The expansion of aluminum is sucked up by more tension in the head bolts, and distortion in the cylinders. Same for the case. The case through bolts constrain the mains. It’s all a very clever steel “cage” around the critical aluminum features. That is the engineering of it.

[Ollies930]

John,
if you use goodcalculators then the result is less than .080", which is a lot less than .125". But that is for pure aluminum, not the alloys we are using. These alloys will grow less than pure aluminum. As an example, Bruce Anderson did a test and heated a Niksil cylinder from 20C to 120C and measured the rate of expansion. What he measured was an expansion of .005" from 3.365" to 3.37". Extrapolating from 3.365" to 36" would give you approximately .054" total expansion.

Speedy Squirrel,
I hope that you are not inferring that the bolts holding the engine together are actually controlling the expansion rate of the Aluminum. All the bolts do, is providing a certain amount of clamping force, which will increase with Aluminums higher expansion rate than steel. But the bolts will stretch to accomodate the growing aluminum and if designed properly, the bolts will stay in their elastic region. If you think that the force applied by the bolts will overcome the force applied by the heat expansion of the aluminum, then I would suggest an experiment with an iron V8 engine block in the winter time. Let the anti freeze out of your cooling system and fill it with water instead. See what happens in a hard freeze.

[winders]

Originally Posted by Ollies930

Speedy Squirrel,
I hope that you are not inferring that the bolts holding the engine together are actually controlling the expansion rate of the Aluminum. All the bolts do, is providing a certain amount of clamping force, which will increase with Aluminums higher expansion rate than steel. But the bolts will stretch to accomodate the growing aluminum and if designed properly, the bolts will stay in their elastic region. If you think that the force applied by the bolts will overcome the force applied by the heat expansion of the aluminum, then I would suggest an experiment with an iron V8 engine block in the winter time. Let the anti freeze out of your cooling system and fill it with water instead. See what happens in a hard freeze.

No, but the increased force applied by the bolts can cause issues with cylinder deformation....

[Ollies930]

Scott,
I agree with your statement, but with a caveat. How much increase in clamping force will cause a significant enough deformation that it will cause ring sealing issues? And at the same time how much deformation are you experiencing with piston side loading pushing against the cylinder walls in comparison? Which is worse?

[winders]

That would depend on rod geometry. One of the reasons Porsche uses dilavar head studs is to reduce the clamping loads the cylinders receive. I am sure that Porsche didn't do this just for fun.

[RobFrost]

Purely a theoretical answer, if you are looking for a cause of the gap shrinking when hot, then the fact the cooling air is applied to the cylinders could cause this. In a steady state engine with everything up to temp, and the cooling applied to the cylinder fins, this could cause the cylinders to be cooler than the rods, crank and pistons, thereby reducing the gap. I doubt this happens in reality though due to oil cooling.

Sent from my SM-G988B using Tapatalk

[stownsen914]

Many variables to consider.
Rod stretch and other flexing/movement as noted above
Bottom end heat expansion, figure 200-250 deg F
Top end heat expansion - exposed to combustion which is much hotter. I believe heads run 300 ish deg F, pistons and cylinders possibly more.
No doubt other factors.

[targa72e]

Thanks Ollies 930.
Math error fixed. more like .22MM per 100C or .080".

john

[Ollies930]

Scott,
the reason Porsche started using Dilavar for head studs were the magnesium engine cases. Magnesium does not require high clamping forces to pull the threads out. Which also explains the extremely low torque value on the head studs.
Realistically Dilavar is not needed on aluminum cases. If you believe otherwise, look at all the motorcycle engine cases and now modern automotive engine cases that have had no problems with steel studs. As far as bore distortion is concened, look at all the Busas, Gixxers and ZX14s with big turbo kits that use APE or ARP racing studs with higher than stock torque values. No problems with cylinder deformation and boost/horsepower keep climbing every year.

[winders]

So you are saying Porsche kept using Dilavar heads studs because they forgot they switched to aluminum cases?

Let's not compare water-cooled inline-4 motorcycle engines to air-cooled 911 engines. The cylinder blocks are MUCH stiffer than the individual 911 cylinders. Also, these engines have much better rod ratios than a typical 911 engine.

[Ollies930]

We are not talking about rod ratios, even though those are all over the place with vintage and modern engines. And you can leave out all the water cooled engines, if you like. How about anyair cooled CB, KZ, GS, Triumph, Vincent, Harley, etc. Aside from that, water cooling does not prevent cylinder growth. It just removes heat better from metal than air, thereby managing to keep parts in a narrower temperature range.
Dilavar studs on paper seem like a good idea, but they brought a reasonably high failure rate with them. I have never heard of any other head stud failing at the same rate.
If you observe the ridiculously low head torque for Dilavar studs, you are limited to about 600-650hp max (not accounting for detonation), before cylinder pressures lift the heads. If you are willing to limit your HP to below that and want to inspect your engine for broken head studs on a regular basis, go ahead and use them.
But with all the other air cooled engines out there, can you tell me why Porsches are the only ones that need Dilavar to keep things together, when everyone else is perfectly fine with steel studs?
But since you mentioned water cooled engines, just to come back to piston to head clearance, as a good example I had a ZX11 that needed .035"-.036" clearance with a stock crankshaft, but when I put in a 5mm stroker crankshaft with the same pistons and rods, the same engine needed .040" minimum clearance. It is all about the internals deforming at high rpm, due to massive Gs being generated by the stroke of the crankshaft.

[winders]

Oh, so now you are saying that Porsche engineers are stupid. Got it.....

[Ollies930]

What I am saying is that even Porsche engineers can make mistakes and if someone up high in the main office tells them to make it work because they have already spent a fortune on material research, then they will try to make it work.
But rather than deflect, explain to me why no one else needed Dilavar, when the engineering and mechanical guidelines remained the same. And if Porsche engineers always got it right, then why so many different Dilavar stud designs.
And how come the IMS shafts were failing in 996s and 997s when these were designed with finite element analysis computers. No one is infallible.